The present invention relates to an amplifying type solid state image sensor device for amplifying signal charge obtained by a photoelectric conversion element such as a photodiode, and in particular to a solid state image sensor device in which an optical black pixel region for defining an optical black level is improved.
Recently, a solid state image sensor device using a CMOS sensor has been proposed as one of solid state image sensor devices. This type of solid state image sensor device is a device wherein a signal detected by a photoelectric conversion element (photodiode) in each of cells is amplified by a transistor. Specifically, in this type of solid state image sensor device, the electrical potential in a signal charge accumulating portion is changed by signal charges generated by the photoelectric conversion element and the electrical potential is amplified by an amplifying transistor in each pixel. Such a solid state image sensor device is expected as an image sensor suitable for making the size of each pixel small.
FIG. 10 shows an example of such a kind of CMOS image sensor in the prior art. In FIG. 10, reference numerals 1, 2, 3 and 10 represent an image sensing cell array, a vertical shift resistor, a horizontal shift resistor, and a timing signal generator, respectively. A plurality of unit cells 13 are arranged in the image sensing cell array, i.e., an image sensing cell area 1. Each of the unit cells 13 is composed of a photodiode 8, a read transistor 14, a driver transistor 15 of a source follower circuit, an address transistor 16, and a reset transistor 17.
Signal charges obtained in each of the unit cells 13 are read out as a signal on a vertical signal line 18 (typically showing any of vertical signal lines 18-1 to 18-(n+m)), supplied to a noise cancellor 25 (typically showing any of noise cancellors 25-1 to 25-(n+m)) through the vertical signal line 18. The signal supplied to the noise cancellor 25 through the vertical signal line 18 is temporarily stored in the noise cancellor 25. The noise cancellor 25 is composed of an S/H transistor 19, a signal transmitting capacitor 20, a signal accumulating capacitor 21, and a clamp transistor 22. The signal temporarily stored in the noise cancellor 25 is read out on a horizontal signal line 26 through a horizontal reading transistor 23, and then is outputted from the image sensor device through an output amplifier 27. In FIG. 10, reference numbers 12, 28 and 20 represent a load transistor, a horizontal reset transistor, and a detecting portion of the signal charges accumulated in the photodiode 8, respectively.
Most of the unit cells 13 constitute photo-sensitive pixels for sensing an image, on the other hand, a part of the unit cells 13 constitutes optical black pixels 13′ for shielding an incident light and generating an optical black signal. Photodiodes of the optical black pixels 13′ are covered with a layer made of aluminum or the like to prevent light from being projected onto the photodiodes. In the same manner as in the photo-sensitive pixels 13, the vertical signal lines 18-n to 18-(n+m), the noise cancellors 25-n to 25-(n+m), and horizontal reading transistors 23-n to 23-(n+m) are provided in the optical black pixels 13′.
An optical black pixel region composed of the optical black pixels 13′ is necessary for defining an optical black level of an obtained signal, i.e., a dark current. That is, in any solid state image sensor device, electrical charges will be thermally generated in semiconductors constituting the device even when any light is not projected onto the device. The thermally generated electrical charges, together with signal charges generated by light projection, are accumulated in photodiodes of the respective pixels. Therefore, in order to detect only the signal charges generated based on the light projection, it is necessary to provide pixels where light is shielded and determine components of the thermally generated charges.
In CMOS image sensors, as shown in FIG. 11, there arises a variation in level of the signals based on dark currents generated in the pixels. In CMOS image sensors, generally, the variation in optical black levels is about 2 mV. The following will describe a case that signals having such a variation are processed in a signal processing circuit of an image sensor, as shown in FIG. 12.
An output from an image sensor 70 has a waveform as shown in FIG. 13A on a signal line 73. The output from the image sensor 70 is composed of signals 78 from photo-sensitive pixels and signals 79 from optical black pixels, and has a reset level 76 and a signal level 77. FIG. 13B shows a signal waveform on a signal line 74, sampled by CDS (Correlation Double Sampling) circuit 71. Since the CDS circuit 71 is a circuit for obtaining the difference between the reset level 76 and the signal level 77, only the signal difference is obtained as the output-signal waveform of the CDS circuit 71. A clamp circuit 72 clamps a signal 79 from the optical black pixels (optical black level 79). A clamped signal waveform is shown in FIG. 13C.
However, such a kind of CMOS image sensor has the following disadvantage. Since there is a variation in levels of the signals generated by dark currents in the optical black pixels, optical black levels in the horizontal lines vary when the clamp circuit 72 clamps signals from the photo-sensitive pixels. This variation also causes a variation in a fixed pattern noise in each horizontal line.
As describe above, in amplifying type solid state image sensor devices using a CMOS sensor in the prior art, there is a variation in optical black signals obtained in the optical black pixels of the optical black pixel region for defining the optical black level. Thus, there remains a problem that a fixed pattern noise in each horizontal line is generated.